If you buy pure pg and vg, that 100 mg nic I linked to, and then got some sort of way to measure it (graduated cylinder, pipettes, etc) it'd be simple.
For a batch of 400 ml total of 75/25 pg/vg, using 100mg/ml nic
Use:
1: 100ml of your nicotine base (100mg/ml)
2: 100ml of vg
3: 200ml of PG...